1. Field of the Invention
This invention pertains, in general, to the field of retractable covers or roofs for large structures, such as athletic stadiums. More specifically, the invention relates to an improved transport mechanism for such a structure that is more compact, reliable, stable, mechanically simple and inexpensive to construct than comparable mechanisms heretofore known.
2. Description of the Related Technology
It is common these days for athletic stadiums to be constructed with retractable roofs, because this type of construction offers spectators the pleasure of being outdoors on pleasant days, while providing shelter when necessary against extreme temperatures and inclement weather conditions. In addition, retractable roof construction permits the use of natural grass in a stadium, which is important to many athletes.
A number of factors must be taken into account in the design of a stadium that has a retractable roof. For instance, the forces created by the exertion of natural forces such as wind, rain snow and even earthquakes on such a large structure can be significant and unpredictable, and the roof, the underlying stadium structure and the transport mechanism that is used to guide and move the roof between its retracted and operational positions must be engineered to withstand the worst possible confluence of such forces. In addition, for reasons that are both aesthetic and practical, it is desirable to make the structural elements of the roof and the transport mechanism to be as unobtrusive and as space-efficient as possible. It is desirable to make the roof structure and the transport mechanism to be as simple and maintenance-free as possible, and to be constructed so as to be able to open and close as quickly as possible.
Many cities in the United States and elsewhere are now using or building convertible stadiums that have retractable roof panels. The designs of the various stadiums that have been built and proposed are quite different, but there are a number of deficiencies that seem to be common to many of the designs that have been implemented thus far. For example, the transport mechanisms in most of the stadiums tend to be quite large, being as much as twenty to thirty feet in height. The transport mechanisms further tend to include a relatively small number of very large, heavily loaded wheels and bearings, and a small number of very large motors or actuators to drive the roof between the retracted and operational positions. The small number of large wheels exert very large concentrated loads onto the support structure, which requires the support structure to be heavily reinforced, adding to the cost and complexity of the stadium as a whole. The roof and transport mechanisms in existing designs further tend to be relatively heavy and inflexible, and often experience alignment problems during movement. While many of these problems have been eliminated through the efforts of Uni-Systems, LLC, as is described in U.S. Pat. Nos. 6,082,054; 6,367,206; and 6,415,556, all to Silberman et al., the disclosures of which are hereby incorporated as if set forth fully herein, additional improvements are possible and are sought after by Uni-Systems, Inc. and others.
When large retractable roofs are built onto stadiums, shipyards or other large buildings, it is difficult to establish two perfectly parallel raceways or tracks on which the roof will rest and travel. The difficulty is trying to get the two tracks perfectly parallel. There are a number of reasons why they may vary in width, including (1) variation from the construction itself; (2) variation from expansion and contraction of the building foundations and walls; (3) variation from external wind forces and/or ice and snow that might develop on one side or the other of the wall; (4) variation from the retractable roof itself which is the result of expansion and contraction from thermal variations; (5) variation from dead-load camber from within the spanning truss and/or (6) variations from racking. It is very difficult to keep a square or rectangular roof perfectly square as it travels down a pair of tracks; this tends to create variable dimensions. The inventors have determined that all of these variations can be accommodated by a properly designed and constructed lateral or horizontal release mechanism.
U.S. Pat. No. 6,415,556 to Silberman et al. discloses a system having a four bar linkage as orientation structure for maintaining the transport mechanism in a predetermined orientation while permitting a limited amount of movement of the roof member in a direction that is nonparallel to the direction of transport. The four-bar linkage does a good job of providing this orientation function if there is sufficient vertical height within the retractable roof to accommodate the four-bar linkage. Key to any orientation mechanism is its ratio of horizontal drag as compared to the vertical gravity component.
For example, if a roof weighs 1,000 tons then the reaction if the sidewall desires to move will be restrained by the coefficient of friction of whatever lateral release mechanism has been utilized. In the case of the four-bar linkage, as long as the linkage bars are exactly straight, up and down vertical, aligned with gravity, the resulting side force from the 1,000 ton roof would be zero. However, as soon as either the roof expands, or the wall line contracts, an angle develops between the four-bar linkage struts. While the struts remain parallel they become tilted when compared to a straight line aligned with gravity. This tilt results in a horizontal thrust or reaction. That reaction becomes more severe as the displacement continues to grow. At some point the angle of the parallel bar linkage becomes too great and the horizontal thrust on the wall and/or roof becomes unacceptably high. The only way to correct this with a four-bar linkage is to extend the length of the parallel bars, which in some cases is objectionable for architectural reasons and the simple geometry of the installation. Accordingly, on many roofs there just isn't sufficient room to use the parallel bar or four-bar linkage orientation system.
A need exists for an improved design for a retractable roof and transport mechanism that is compact, lightweight and mechanically simple, and that is capable of maintaining its stability and alignment during normal use and in extreme conditions more capably than comparable mechanisms heretofore known.